Halogen exchange reactions for fluorinating haloaromatic compounds using alkali metal fluorides as the fluorine source have been extensively studied heretofore. Typically they involve the reaction of a chloroaromatic compound with potassium fluoride, rubidium fluoride or cesium fluoride by heating the reactants to extremely high temperatures (above about 400.degree. C.) in the absence of an ancillary diluent or solvent, or by conducting the reaction at temperatures of around 200-230.degree. C. in an aprotic solvent such as sulfolane. It has also been reported that organic fluorine compounds such as pentafluorobenzonitrile, tetrafluorophthalonitriles and pentafluoropyridine can be formed by reacting a corresponding chloro- or bromo-substituted compound with alkali metal halide such as potassium fluoride in benzonitrile as solvent at 190.degree. C. to 400.degree. C. in a sealed autoclave under autogenous pressure.
Use of catalysts in some exchange reactions has also been studied. Such catalysts have included quaternary ammonium salts, metal carbonyls, crown ethers and cryptates. In now commonly-owned application Ser. No. 08/754,338, filed Nov. 22, 1996, now U.S. Pat. No. 5,824,827, all disclosure of which is incorporated herein by reference, Igor Bildinov et al. describe a significant improvement in halogen exchange technology, namely that aminophosphonium compounds such as one or more tetra(dihydrocarbylamino)phosphonium halides are highly effective catalysts for halogen exchange reactions whether the reaction is conducted as a mixture of solids or as a slurry.
Aminophosphonium catalysts, such as tetra(dihydrocarbylamino)phosphonium halides, although highly effective as catalysts, are nonetheless relatively expensive materials. Thus it would be highly desirable to recover such catalysts for reuse as catalysts. However, the formation of heavy ends during halogen exchange reactions complicates the recovery of such catalyst components in suitably pure, catalytically active form. Thus a need has existed for an effective way of recovering such catalysts in catalytically active form enabling reuse of such materials, especially as halogen exchange catalysts. This invention is deemed to fulfill this need most expeditiously.